The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Plasma etching is frequently used in semiconductor fabrication. In plasma etching, ions are accelerated by an electric field to etch exposed surfaces on a substrate. The electric field is generated based on RF power signals generated by a radio frequency (RF) generator of a RF power system. The RF power signals generated by the RF generator must be precisely controlled to effectively execute plasma etching.
A RF power system may include a RF generator, a matching network, and a load (e.g., a plasma chamber). The RF generator generates RF power signals, which are received at the matching network. The matching network matches an input impedance of the matching network to a characteristic impedance of a transmission line between the RF generator and the matching network. This impedance matching aids in maximizing an amount of power forwarded to the matching network (“forward power”) and minimizing an amount of power reflected back from the matching network to the RF generator (“reverse power”). Forward power may be maximized and reverse power may be minimized when the input impedance of the matching network matches the characteristic impedance of the transmission line.
In the RF power source or supply field, there are typically two approaches to applying the RF signal to the load. A first, more traditional approach is to apply a continuous wave signal to the load. In a continuous wave mode, the continuous wave signal is typically a sinusoidal wave that is output continuously by the power source to the load. In the continuous wave approach, the RF signal assumes a sinusoidal output, and the amplitude and/or frequency of the sinusoidal wave can be varied in order to vary the output power applied to the load.
A second approach to applying the RF signal to the load involves pulsing the RF signal, rather than applying a continuous wave signal to the load. In a pulse mode of operation, a RF sinusoidal signal is modulated by a modulation signal in order to define an envelope for the modulated sinusoidal signal. In a conventional pulse modulation scheme, the RF sinusoidal signal typically is output at a constant frequency and amplitude. Power delivered to the load is varied by varying the modulation signal, rather than varying the sinusoidal, RF signal.
In a typical RF power supply configuration, output power applied to the load is determined by using sensors that measure the forward and reflected power or the voltage and current of the RF signal applied to the load. Either set of these signals is analyzed in a typical control loop. The analysis typically determines a power value which is used to adjust the output of the RF power supply in order to vary the power applied to the load. In a RF power delivery system, where the load is a plasma chamber, the varying impedance of the load causes a corresponding varying power applied to the load, as applied power is in part a function of the impedance of the load.
In plasma systems, power is typically delivered in one of two configurations. In a first configuration, the power is capacitively coupled to the plasma chamber. Such systems are referred to as capacitively coupled plasma (CCP) systems. In a second configuration, the power is inductively coupled to the plasma chamber. Such systems are typically referred to as inductively coupled plasma (ICP) systems. Plasma delivery systems typically include a bias power and a source power applied to one or a plurality of electrodes. The source power is typically used to generate the plasma and the bias power is typically used to tune the plasma to an energy level relative to a bias RF power level. The bias and the source may share the same electrode or may use separate electrodes, in accordance with various design considerations.
When a RF power delivery system drives a load in the form of a plasma chamber, the electric field generated by the power delivered to the plasma chamber results in ion energy within the chamber. The ion energy is distributed non-uniformly due to particle drift/diffusion effects and the externally applied fields. One characteristic measure of ion energy is the ion energy distribution function (IEDF). The ion energy distribution function (IEDF) at the substrate surface can be controlled with a RF waveform. One way of controlling the IEDF for a system in which multiple RF power signals are applied to the load occurs by varying multiple RF signals that are related by frequency and phase. The frequencies between the multiple RF power signals are locked, and the relative phase between the multiple RF signals is also locked. Examples of such systems can be found with reference to U.S. Pat. No. 7,602,127, U.S. Pat. No. 8,110,991, and U.S. Pat. No. 8,395,322, assigned to the assignee of the present invention and incorporated by reference in this application.
RF plasma processing systems include components for plasma generation and control. One such component is referred to as a plasma chamber or reactor. A typical plasma chamber or reactor utilized in RF plasma processing systems, such as by way of example, for thin-film manufacturing, utilizes a dual frequency system. One frequency (the source) of the dual frequency system controls the generation of the plasma, and the other frequency (the bias) of the dual frequency system controls ion energy. Examples of dual frequency systems include systems that are described in U.S. Pat. No. 7,602,127; U.S. Pat. No. 8,110,991; and U.S. Pat. No. 8,395,322 referenced above. The dual frequency systems described in the above-referenced patents include a closed-loop control system to adapt RF power supply operation for the purpose of controlling ion density and its corresponding IEDF.
Multiple approaches exist for controlling a plasma bias potential and thus the corresponding IEDF. The approaches include: conventional low-frequency sine wave biasing; multiple sine wave biasing without harmonic locking; harmonically locked multiple sine wave biasing; and shaped biasing to create a monotonic or custom IEDF. Each of these approaches has associated disadvantages and/or limitations. For example, the low-frequency sine wave biasing approach exhibits a bimodal IEDF (i.e. primarily two non-zero ion energy levels or absolute voltage potentials over a cycle of a RF bias voltage signal). A monotonic IEDF (i.e. primarily a single non-zero ion energy levels or absolute voltage potential over a cycle of a RF bias voltage signal) is better than a bimodal IEDF for controlling etch profiles and/or etch selectivity.
As another example, the harmonically controlled multiple sine wave biasing approach may be implemented in high-power implementations, but includes the use of large expensive generators, which are difficult to isolate from each other when operating at low bias RF frequencies. The harmonically controlled multiple sine wave biasing approach experiences instantaneous high voltage peaks or nulls that affect instantaneous ion energy levels due to variation in instantaneous plasma sheath voltage.
The shaped biasing approaches can be power limited and also includes large expensive power generators. One shaped biasing approach includes a broadband amplifier. It is difficult to properly match a non-linear plasma impedance to a source impedance of the broadband amplifier. In addition, a broadband amplifier is typically power inefficient and expensive.
Another shaped biasing approach includes the use of a switch mode power supply and current source for generating a bias potential. This approach includes pulse width modulation and sine wave modulation and is power and voltage limited due to use of transistors in a half-bridge configuration and corresponding breakdown voltages of the transistors. Only one of the transistors is ON at any moment in time. It is also difficult to scale this approach for high power applications, such as high aspect ratio (HAR) plasma etch processes.
While the above systems enable a certain degree of control of a plasma process, the continually increasing need for smaller components and increased yields demand continual improvement over the above-described approaches.